When cancer is detected early enough, surgery alone can often be curative. However, by the time of diagnosis, the cancer has frequently spread to other sites in the body, by the process of metastasis. Much of the pain, suffering, and mortality associated with cancer are the result of metastatic disease. We need better ways to diagnose metastatic disease early, and better ways to treat metastatic disease therapeutically. The ultimate goal is to prevent metastasis from occurring, particularly in those who have been diagnosed with pre-invasive conditions such as ductal carcinoma in situ of the breast, and prostate intra-epithelial neoplasia. We will achieve this goal when we are able to define and develop therapeutics against targets present in metastatic cells which are not essentially required for most normal physiology, thus allowing for safe and effective long term treatment. Both metastatic and benign tumors proliferate without regard to the cellular cues that normally control cell division and cell death. But only the metastatic cells have the ability to break free from the organ in which they arise, enter the vasculature, and move into and grow in other organs. These steps are collectively called invasion. It is widely believed that the most common way in which invasion occurs during metastasis is through the action of proteases, which degrade extracellular matrix (ECM) proteins. Since continued ECM degradation is also thought to be required for disseminated tumor growth in secondary organ sites, inhibiting this process may provide a means to control metastasis. Cancer cells frequently over-express a number of proteases that are capable of degrading ECM. Some of these proteases are secreted, while others remain associated with the plasma membrane. In recent years, it has become increasingly clear that specialized protrusions of the ventral membrane of cancer cells called podosomes (also known as invadopodia) are the concentrated sites of action of these proteases. For example, using a model system of invasion, mouse fibroblasts transformed by an activated version of the Src oncogene, we have shown that podosomes are required for the invasiveness that typifies Src transformation, both in vitro and in vivo. Cells lacking podosomes still secrete proteases, but they are unable to degrade ECM efficiently. Furthermore, human cancer cells also require podosomes/invadopodia for invasion. These results are consistent with a model in which podosomes/invadopodia are required for the coordinated activity of proteases. Further, these data support our hypothesis that small molecules that prevent the formation of podosomes would be effective agents to limit the growth of tumor metastases. Here, we propose to develop and utilize cell-based, high throughput screening systems to identify chemical entities that inhibit podosome formation and invasion. Metastasis is the leading cause of cancer mortality in this country. We need better ways to diagnose metastatic disease early, and better ways to treat metastatic disease therapeutically. The proposed research aims to set develop high throughput screening assays to identify inhibitors of metastasis, and therefore represents the first step to developing anti-metastatic drugs with a new mechanism of action.